There is widely used a disk array that makes memory devices (hard disk drives (HDD), solid state drives (SSDs), or the like) redundant by using a redundant arrays of inexpensive disks (RAID) technology or the like and that realizes high-speed data access and enlargement of capacity. On the other hand, if the number of the memory devices is increased in order to increase the capacity of a disk array, the amount of electric power consumed by the disk array is increased. Therefore, from the viewpoint of reducing a running cost or an environmental load, research and development regarding power saving of a disk array have been advanced.
As a technology for power saving of a disk array, there is, for example, a massive array of idle disks (MAID) technology. The MAID technology is a technology for reducing power consumption by causing a memory device to make a transition to a power saving mode (for example, reduction or stopping of the number of rotations of a disk, retracting of a disk head, power-off, or the like), in which the memory device is included in the disk array and no data access to the memory device is generated. In a case where a data access to a memory device in a state of the power saving mode is generated, a controller to control memory devices restores, to a normal operation mode, the memory device subjected to the data access.
If a time period for maintaining a memory device in a state of the power saving mode is long, an effect for suppressing power consumption is increased by an amount corresponding thereto. Therefore, in a case where the data access to the memory device in a state of the power saving mode is generated, possibility of normally processing the data access while not restoring the memory device to the normal operation mode contributes to further power saving.
Note that there is proposed a technology for providing an SSD in a disk array including HDDs and writing data into the SSD in a case where a write request to an HDD whose rotation of a disk is stopped is received from a host computer. If a status of writing data into the SSD satisfies a given condition, this disk array rotates the disk of the corresponding HDD and transfers data of the SSD to the HDD. In addition, there is proposed a technology for writing data into an HDD in a case where another inactive HDD included in HDDs configuring a RAID is requested to write.
As examples of the related art, Japanese Laid-open Patent Publication No. 2013-206009 and Japanese Laid-open Patent Publication No. 2011-238038 are known.